Space vehicles and aircraft have infrared sensor systems that scan the Earth or space and detect infrared radiation generated from Earth or in space to track missiles, measure remote temperatures, and view night scenes. Infrared radiation detected by these sensor systems includes target radiation and background radiation. Missiles, growing crops, and humans comprise targets which generate generally changing intensities of infrared radiation relative to generally constant background radiation that stars in space and the earth generate, for instance.
Infrared sensor systems include arrays of radiation detectors that sense infrared radiation and generate output signals depending on the intensity of detected infrared radiation. The radiation detectors are also responsive to high energy gamma rays that frequently occur in space. The radiation detectors generate very short duration, high amplitude signals, known as gamma spikes, in response to the high energy of the gamma rays. However, it is undesirable for infrared radiation detectors to respond to gamma rays in this manner, because gamma spikes complicate the accurate measurement of intensity of detected infrared radiation.
Infrared sensor systems typically have an optical portion that focuses radiation onto infrared detectors mounted in a focal plane of that optical portion. These infrared detectors each generate an analog signal that indicates the intensity of radiation detected by such a detector. Multiplexors convey these analog signals from the radiation detectors to a processor which analyzes the intensity of detected radiation. Such analog signals tend to interfere with one another. Systems involving analog signals are greatly effected by external interference.
Some infrared sensor systems use analog-to-digital converters to convert the analog signals from radiation detectors into digital signals. These digital signals are much less likely to interfere with one another. Systems involving digital signals are effected by external interference much less than systems involving analog signals. Despite the advantages of systems involving digital signals over systems involving analog signals, analog-to-digital converters consume a great deal of electrical power, the supply of which is limited aboard space vehicles and aircraft.
Current infrared sensor systems have approximately 40 thousand infrared detectors and multiplexors sample each detector at a rate of 10 kilohertz. It has been estimated that by 1995 arrays in such systems will have 1 million detectors and multiplexors will sample each detector at a rate of 30 kilohertz. Resulting circuitry will be more susceptible to interference and will consume more electrical power.
Thus, a need exists, which will be critical in the future, for a circuit that produces signals representing intensity of detected infrared radiation in a form that is not susceptible to interference and in a way that does not consume a great deal of electrical power.
FIG. 1a shows a scanning radiation sensor system 20. The sensor system 20 scans space or the Earth for radiation generated from a target by moving a lens 21 together with a plurality of radiation detectors 22, for instance. The lens 21 receives background and target radiation from an instantaneous field of view that is typically one degree horizontal and sixty degrees vertical. The lens 21 collects the background and target radiation and focuses numerous glow spots 23 of radiation toward the radiation detectors 22. Each glow spot 23 of radiation corresponds to a single target or background object, such as a single star in the instantaneous field of view of the lens 21. Each glow spot 23 is slightly larger than one radiation detector. Thus, the lens 21 focuses a glow spot 23 of radiation onto a set of radiation detectors comprising a central radiation detector and any radiation detectors which are immediately adjacent that central radiation detector. The glow spots 23 from numerous targets or background objects move horizontally over the radiation detectors 22 as the sensor system 20 moves the lens 21 together with the radiation detectors 22 left and right in an arc of 120 degrees, and are positioned vertically on the radiation detectors 22 according to the elevation of each target or background object in the field of view, for example. Movement of these glow spots 23 describes a surface known as a focal plane 24.
Typically, modules 22a of radiation detectors 22 are mounted in columns which are staggered vertically on a surface in the focal plane 24 of the moving lens 21. These columns of modules 22a are staggered vertically so that any ray of radiation which might pass undetected through a horizontal space between two vertically adjacent modules in a first column, hits another module in a horizontally adjacent column.
Each radiation detector of each module generates an analog output signal that indicates intensity of radiation detected by such a detector. The analog output signal of each radiation detector is processed in a processor to derive intensity of radiation generated by each target or background object and angular position of that target or object during an initial scan of the glow spots 23 by the lens 21 over the radiation detectors 22. The processor then compares a subsequent intensity of radiation and angular position for each target or object derived during a subsequent scan of the glow spots 23 to determine a trajectory of a target or position of a background object, for instance.
FIG. 1b shows radiation detectors 22 of a scanning radiation sensor system 20 in a line array 25. Typically, line arrays 25 of radiation detectors 22 are mounted in two vertical columns 26 and 27 on a surface in a focal plane 24. The sensor system 20 moves the lens 21 together with the radiation detectors. As a result, glow spots of radiation are scanned horizontally over these two columns 26 and 27. The two columns 26 and 27 of radiation detectors 22 in the line array 25 are staggered so that any ray of radiation, which might pass undetected through a horizontal gap 28 between two adjacent radiation detectors 22 in a first column 26, hits another radiation detector 22 in a second column 27, and is detected. Radiation detectors 22 of each column 27 generate output signals representing intensity of detected radiation.
FIG. 1c shows radiation detectors 22 of a scanning radiation sensor system 20 in a mosaic array 29. Typically, mosaic arrays 29 of radiation detectors 22 are mounted on a surface in a focal plane 24 of the sensor system 20. These mosaic arrays 29 of infrared detectors typically comprise six vertical columns 30, 31, 32, 33, 34, and 35 in each of two array sections 36 and 37. Glow spots of radiation are scanned horizontally over the six columns of one section 36 and then over the other section 37. Radiation detectors 22 of each column generate output signals representing intensity of detected radiation.
A time delay-and-sum circuit, not shown in the figures, incrementally delays sequential output signals from radiation detectors 22 in these six columns until the output signals of radiation detectors 22 in all these six columns are in phase. This delay depends on rotational speed of the lens 21 as it scans and a known angle-separation between columns of adjacent radiation detectors. The delay-and-sum circuit then sums the output signals of these radiation detectors 22 to improve signal-to-noise ratio by the square root of the number of columns. This technique is known as time delay integration. The sections 36 and 37 of the mosaic array are staggered so rays of infrared radiation do not pass undetected through gaps 28 between radiation detectors 22.
FIG. 1d shows radiation detectors of a staring radiation sensor system 20 in a mosaic array 38. Typically, mosaic arrays 38 of radiation detectors 22 are mounted on a surface behind a lens that remains stationary for surveillance of scenes which generate infrared radiation. Such a lens focuses radiation at different angles onto individual radiation detectors 22 of a staring sensor system 20. Output signals from individual radiation detectors 22 are time-integrated to indicate intensity of detected radiation.
The radiation detectors 22 in a sensor system of FIG. 1a, 1b, 1c, or 1d not only detect infrared rays, but also respond to undesirable gamma rays. These detectors produce analog output signals indicating intensity of radiation caused by both types of these rays. Thus, a sensor system for detecting infrared radiation and comprising such radiation detectors 22 must compensate for the presence of undesirable gamma rays.
FIG. 2 shows one type of radiation detector circuit that reduces effects of undesirable gamma rays sensed by a radiation detector. However, this circuit inaccurately indicates intensity of detected radiation. A detector 22 detects infrared radiation and generates an analog output signal that indicates intensity of that radiation to a wide band amplifier 39. This wide band amplifier 39 passes an amplified output signal through a closed switch 40 to an integrator 41. The integrator 41 generates an integrated output signal to a sample-and-hold circuit 42. The sample-and-hold circuit 42 periodically samples the integrator 41 output signal and holds that sample. A multiplexor 43 reads out samples representing detected radiation from a plurality of sample-and-hold circuits 42 in other radiation detection circuits and sends those samples to a processor 44. The processor 44 demultiplexes the samples and derives intensity of detected infrared radiation from these samples. The sample-and-hold circuit 42 also resets the integrator 41 to obtain subsequent samples.
The detector 22 of FIG. 2 generates a gamma spike comprising a high amplitude output signal when the detector 22 detects a gamma ray. The wide band amplifier 39 passes this gamma spike to a differentiator circuit 45. The differentiator circuit 45, on sensing the gamma spike, opens the switch 40 from its closed position to prevent the gamma spike from passing to the integrator 41. However, the open switch 40 also prevents output signals that represent infrared radiation from passing to the integrator 41. Gamma spikes occur frequently, causing this circuit to open frequently and inaccurately detect radiation.
FIG. 3 shows another type of radiation detector circuit that compensates for the presence of undesirable gamma rays, but inaccurately indicates detected radiation. A detector 22 detects infrared radiation and generates an analog output signal that indicates intensity of that radiation to a wide band amplifier 39. This wide band amplifier 39 passes an amplified output signal to an integrator 41. This integrator 41, sample-and-hold circuit 42, multiplexor 43 and processor 44 function the same as those of FIG. 2.
The detector 22 of FIG. 3 generates a gamma spike comprising a high amplitude signal to the wide band amplifier 39 when a detector 22 detects a gamma ray. The gamma spike saturates the wide band amplifier 39 which generates a constant voltage output signal during the gamma spike. The integrator 41 integrates this constant voltage output signal with analog signals which represent detected infrared radiation. Gamma spikes occur frequently, causing this circuit to saturate frequently and inaccurately detect radiation.
The circuits of FIGS. 2 and 3 generate analog output signals to indicate intensity of detected radiation. Such analog signals are very susceptible to interference. Digital signals are less susceptible to interference and are immune to gamma noise, for instance. Multiplexors multiplex digital signals much faster and with greater accuracy than analog signals. Thus, these circuits require analog-to-digital converters to convert these analog signals to digital signals for fast multiplexing without interference. However, analog-to-digital converters consume a great deal of electrical power.